Electron Transfer (ET) reactions play a central role in chemical and biological processes. Early experimental and theoretical studies have identified the key factors in ET reactions and reach a qualitative understanding using a continuum model for the solvent. However, recent experiments are providing detailed information on the level where the microscopic nature of the actual environment of the donor and acceptor might be very important. The basic objective of this proposal is to contribute toward a more microscopic description of ET reactions in solution and proteins using actual molecular simulation. It is proposed to simulate ET reactions in several key experimental systems, focusing on the detailed correlation between the theoretical and experimental results. Our proposed simulation studies will include the following projects: (i) Studies of the microscopic correlation between activation from energies and the corresponding solvent reorganization energies and free energies for ET in solution and in proteins. Significant effort will be dedicated to studies of chemically modified proteins (e.g. Ru-cytochrome c). (ii) Microscopic simulation of dynamical effects in ET reactions, with the goal of correlating rate constants with the dielectric relaxation times of the given microenvironments. (iii) Calculations of the intramolecular Franck-Condon activation barriers in highly exothermic reactions. (iv) A major effort will be dedicated to studies of the detailed energetics and dynamics of ET processes in bacterial photosynthesis. This project will try to use the X-ray structure or the reaction center to evaluate the energies of relevant charge transfer states and to explore the effects of the fluctuations of the protein dipoles. (v) The redox potential of electron transfer proteins will be evaluated by free energy perturbation methods. This study will include calculations of the effects of genetic modifications on redox potentials.